1. Field of the Invention
This invention relates generally to the field of controllable heat pipes and, more particularly, to controllable heat-valves which are controllable by a control temperature.
2. Description of the Prior Art
Heretofore, most of the commercial applications of heat pipes and heat siphons have concentrated either on the removal of heat from places that are inaccessible to the passage of a cooling fluid, as in the cooling of concentrated arrays of electronic equipment or heat removal from interior regions during die casting, or on the transfer of heat from a region where it is harmful or wasted to a region where it can be put to good use, as in the case of heat transfer from the hot sunlit side of a spacecraft to the cold shadow side or the transfer of waste chimney heat to a site where it can be used for space heating or industrial processing. These applications are characterized by the fact that the heat pipe must function continuously as long as the heat source generates heat. The requirement to maintain the constancy of the temperature at which electronic equipment operates has also led to the invention of sundry methods for causing the effective thermal conductance of a heat pipe to vary automatically in such a way as to cause a diminishment in the variations in temperature of the heat source to which the evaporator end of the heat pipe is attached. The most common approach involves the inclusion of a quantity of inert gas in the heat pipe or in a connecting reservoir in such a way that it competes with the working fluid vapor arriving from the evaporator for access to the condenser region of the pipe. (See ref. 1 for details.)
An entirely different category of applications is, however, becoming feasible that requires the capability of switching heat pipes on and off and regulating their effective thermal conductances continuously between these extremes. For example, it is well known that it is possible to store a great quantity of high-temperature heat energy in the form of metallic vapor contained in an insulated tank, but to tap this energy for metallurgical or other industrial processing, or for conversion into mechanical or electrical energy by means of a Stirling engine, it is necessary not only to transport the heat from the reservoir where it is stored to the site where it is needed, but also to be able to turn the heat supply on and off in a manner analogous to the control of the flow in a hydraulic line by means of a valve inserted in the line. There have been some inventions that modify heat pipes so as to provide this capability (reviewed in ref. 1), and these can be divided into two categories: those that require the availability of electrical energy, and those that can operate from the same heat supply that feeds the heat pipe. When electrical energy is readily available and the immediate environment of the heat pipe is not hostile to the necessary electronic controls, then there exist several satisfactory means for making a heat pipe function as an on-off switch, and, although it is considerably more difficult to provide continuous variation of the effective conductance between the extremes of on and off, certain of the electrical methods can also provide this capability. If, however, electrical energy is not available, or if very high temperatures are involved that require elaborate shielding and cooling of the electronic control equipment, or if the heat pipe is used to conduct heat from a nuclear reactor in the proximity of which electronic equipment would soon be destroyed because of radiation, or if the heat valve is part of a system that must operate unattended for long periods of time in remote or inaccessible locations so that repair or battery replacement is not possible, then it is necessary that the heat valve be powered solely by a heat source, and that it be robust, durable, and reliable. These requirements, in turn, make it very desirable that the heat valve not contain any mechanically moving parts which would be subject to mechanical fatigue. This would eliminate the inclusion of bimetallic strips or liquid filled bellows within the heat pipe for the purpose of opening or closing a break in the wick that returns the condensate to the evaporator (cf. U.S. Pat. No. 3,519,067). This invention, incidentally, intended that the bimetallic strips or bellows be activated by the temperature applied to the evaporator end of the heat pipe so that the device would serve to smooth out variations in the temperature of the heat source, rather than act as a thermal on-off switch.)
A more promising proposal was made by R. D. Moore, Jr. in U.S. Pat. No. 3,818,980. Moore proposed regulating the conductance of the heat pipe by varying the amount of working fluid contained in it. This would be done by connecting the pipe to an external reservoir whose temperature could be independently controlled. If this temperature were below the operating temperature of the heat pipe, the vapor pressure in the reservoir would be less than that in the heat pipe, and vapor would flow from the pipe into the reservoir where it would condense and be held in a wick. Thus deprived of its working fluid, the heat pipe would cease to function. It could be started again by raising the reservoir temperature above that prevailing in the heat pipe, in which case vapor would flow back into the pipe. Because a very small change in temperature suffices to produce a very large change in saturated vapor pressure, the switch between the on and off positions of the heat pipe is accomplished by very small changes in the reservoir temperature, with the result that it is essentially impossible to maintain the heat pipe in some intermediate stage of conductance. Significantly, Moore consistently refers to the "on" and "off" states of his device, although he claims that intermediate states are possible. Even in switching between these two states, there are inconveniences, if not problems. In order to make the switch, it is necessary for the control source that maintains the temperature of the reservoir either to absorb or deliver a rather large quantity of heat, namely the heat of vaporization of a significant fraction of the total inventory of the heat pipe. This in turn slows down the switching operation unless very large changes in the control temperature are employed. There can also be troublesome feedback from the heat pipe circuit: For example, if the pipe is turned off by making the reservoir pressure lower than the pipe pressure, and if the pipe load (i.e. passive thermal resistance through which the pipe current flows) is predominantly on the condenser end, then when the pipe heat current is shut off the condenser temperature, and hence the pressure in the pipe, will drop precipitously. Unless the reservoir has been driven to an even lower temperature and pressure, vapor will flow back into the pipe, turning it on again. Although the source temperature must be capable of large swings, its average value is about equal to the temperature prevailing in the heat pipe. Thus it would not be possible to use a low control temperature to control high-temperature heat flow. Moore did, in fact, describe a means whereby the average control temperature could be lowered, but this involved a second reservoir containing a different, more volatile fluid whose vapor pressure was to be used to control the pressure of the first reservoir, now enclosed in a bellows. This would introduce a fatigue-prone element, the bellows, without removing any of the problems except for the high average control temperature.
The Moore device is to be contrasted with my invention which involves a compact sealed device without any connecting reservoirs or any moving or fatigue-prone parts. The control temperature can be chosen to be any temperature below the operating temperature of the evaporator, and will operate well at temperatures well below that of the condenser, so that a low temperature can be used to control high-temperature heat flow. The quantity of heat that must be delivered or absorbed by the control source, both during steady-state operation and during a change in control temperature, is very small, so a low-conductance control source is feasible. This makes it possible to use the output of a low-power, low-temperature thermal amplifier (described below) as the control source for a high-power, high-temperature heat valve. There is very little internal feedback that would tend to make the effect of the control unpredictable, but it is easy to provide external feedback if desired, in a way that is completely analogous to what is done in vacuum tube and transistor circuits. Similarly, it is easy by external means to change the value of the control temperature that shuts off the heat pipe. Finally, unless external positive feedback is intentionally added in order to provide a sharp and sudden transition between the on and off states, it is very easy to maintain intermediate states of heat pipe conductance.
Because of the cascading that is made feasible by the characteristics of my device, it is possible to make very small changes in the temperature of a target or fin connected to the control grid of the first-stage amplifier cause a very large high-temperature heat valve to trigger on or off. This provides the means for servo-control of large-scale industrial processes that is powered by the same source that provides the heat for the process. It also provides the means for switching from a distance, since with sufficient amplification a light beam pulse focussed on the target that activates the first-stage amplifier can be made to open the large heat valve.
The availability of such controllable heat valves will make feasible the constant unattended accumulation and storage of solar energy (both on earth and in space) over long periods of time for use in short, high-power, high-temperature bursts of thermal energy. This capability would be useful for devices ranging from solar-powered cooking stoves to servo-controlled furnaces for materials processing in space. The use of valve-controlled heat reservoirs would also make the burning of fossil fuels more efficient because it would divorce the burning rate from the variable power demand. Thus the burner could be designed to burn steadily under its most efficient operating conditions at a rate that would suffice to satisfy the average, rather than the instantaneous, power demand.